[GTK] Do not try to remove fixedHandle if there is none

[akurtakov]

GtkWidgets can act as containers thus SwtFixed don't have to used always.

[github-actions]

Test Results

   539 files   -  7     539 suites   - 7   31m 18s ⏱️ + 3m 4s
 4 371 tests  - 54   4 356 ✅  - 52   14 💤  - 3  1 ❌ +1 
16 692 runs   - 54  16 567 ✅  - 52  124 💤  - 3  1 ❌ +1 

For more details on these failures, see this check.

Results for commit 8f70fb6. ± Comparison against base commit e2e8c1d.

This pull request removes 54 tests.

AllWin32Tests ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
AllWin32Tests ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
AllWin32Tests TestTreeColumn ‑ test_ColumnOrder
AllWin32Tests Test_org_eclipse_swt_dnd_DND ‑ testByteArrayTransfer
AllWin32Tests Test_org_eclipse_swt_dnd_DND ‑ testFileTransfer
…

[akurtakov]

Windows test failure is unrelated to GTK only change. Merging.